I don't know for certain but my guess is PHP

Most Hidden Service operators use PHP, I have seen Python and Java a few times. PHP is really the way to go these days, especially because a hidden service is A LOT of the time at the mercy of the Tor Networks health(I.E. Intro-circuit attacks, DDOS, ETC...). Imagine building this super fast, compiled C or the like website, its super fast and everything is optimized, but along comes the Tor Network with its current faults, you are soon going to be frustrated that all your optimization and wheel re-invention didn't quite do the trick and your website is about as responsive as Joe's who did a good job designing a PHP hidden service in about half the time.

I do believe that Dread is written in PHP, obviously Dreads got a lot going on in the background so im sure that its not all that's under the hood.

Take what you will and please correct me where I am wrong. Most hidden service developers and operators are going to use what they are most familiar with and what makes sense. Using frameworks and the like speed up development time, so basically you can look up all frameworks for building websites and get a good idea what all the possibilities are for creation of hidden services, remember they are just websites albeit with out JS. :)

Why are you curious about what software Monopoly used? maybe is you have some specifics we can answer or give you some information that you may be looking for.